1. Field of the Invention
The present invention relates to a method of manufacturing a printed circuit sheet having a two-layer circuit pattern.
2. Description of the Related Art
According to a known conventional method of manufacturing a printed circuit sheet (Japanese Patent Disclosure (Kokai) No. 60-121791), a circuit pattern transfer sheet is placed in an injection molding die, and a circuit pattern is integrally formed on a required surface of an injection-molded body simultaneously with injection molding. The circuit pattern transfer sheet in this method is obtained by printing a circuit pattern on a releasable base sheet by using a conductive paste, and forming an adhesive layer for adhesion with the injection-molded body thereon. In this method, a circuit pattern is formed in a single layer.
As a circuit structure becomes complex, circuit patterns must be formed in two layers. When circuit patterns are to be printed in two layers by using a conductive paste, the following method is employed. A first-layer circuit pattern is printed on a base sheet using a conductive paste, thereafter an insulating layer consisting of an insulating paste, and through studs (for electrically connecting circuit patterns of two layers to each other) are printed overlapping the first-layer circuit pattern. Then, a second-layer circuit pattern is printed on the resultant structure by using a conductive paste.
According to such a manufacturing method, however, since an insulating layer for insulating circuit patterns of two layers is formed by screen printing using an insulating paste, in order to obtain a sufficient thickness (60 .mu.m or more) of the insulating layer, printing must be performed three times or more. As a result, the number of steps and hence the cost are increased. In addition, since printing is performed a plurality of times, variations in layer thickness tend to occur and voids are easily generated. Therefore, reliability of interlayer insulation is insufficient. A silver paste is generally used as a conductive paste. However, if the silver paste is printed on an insulating paste, migration of silver tends to occur.